Your veterinarian can give each cat a product that will stain its urine marks with a color that can be detected by an ultraviolet light. (from the FDA Veterinarian web site)
How Do I Color A Cats Urine So I Can Tell Which Cat Is Peeing? Food Coloring Won't Do It?
I don't know that but you are I presume using multiple cat boxes as some cats refuse to pee where a lot of other animals do, try putting a cat box near the site and other places. Dusting area with pepper helps too as the cats sniff area first when they pee
We are having the same problem with our cats - and we found food coloring does only color the feces. My thinking is -- if the cat DRINKS (like in water) the food coloring - that should pass through the bladder. We are going to try that, and we'll keep you informed.